Multiple channel carrier current telephone cable



May 14, 1957 R E 2,792,442

MULTIPLE CHANNEL CARRIER CURRENT TELEPHONE CABLE Filed July 15, 1955United States Patent MULTIPLE CHANNEL CARRIER CURRENT TELEPHONE CABLELeon Jean Parc, Sceaux, France, assignor to Societe Anonyme deTelecommunications, Paris, France, a corporation of France ApplicationJuly 15, 1953, Serial No. 368,127

Claims priority, application France July 30, 1952 1 Claim. (Cl. 174-32)The present invention relates to a telephone cable for multiple channelcarrier current transmission. In order to transmit over as high a numberof channels as possible, it is necessary, on the one hand, to operatethese cables with constantly higher frequencies, and, on the other hand,to build them with a large number of circuits (pairs or quads),distributed into several cable layers.

It is well known that, to obtain good transmission characteristics insuch a cable, it is necessary that the time of propagation t and thecharacteristic impedance Z of all its circuits be the same. The valuesof these constants are given by the approximate expressions:

where L is the inductance of a circuit per unit length, per kilometerfor instance, and C the capacitance of the same circuit per unit length,per kilometer for instance.

It is also known that the inductance of a given circuit decreases withincreasing frequency, and that the law of variation of inductanceagainst frequency is not the same according to whether the said circuitis placed in the layer close to the lead sheath of the cable or in alayer remote from said sheath, and that this variation is more rapid forcircuits closer to the sheath.

Let L2 and C2 be the inductance and capacitance per unit length of thecircuits in an outer layer, and L1 and C1 the inductance and capacitanceper unit length of the circuits in an inner layer.

In order that both the constants Z and t be equalized, it is obviouslynecessary that L1=L2 and C1=C2. It will be seen from the foregoing, thatit is not possible for the first one of these relationships to besatisfied both at high and low frequencies. However, in practicaltransmission conditions, it is particularly important that thisrelationship be satisfied at high frequencies. So L1 must be less thanL2 at low frequencies. This requires that, according to a known method,the two wires in a circuit or pair in the inner layers be arrangedcloser to each other than the two wires in a circuit in a peripherallayer and at the same time that the capacitances per unit length C1 andC2 for the considered circuits in different layers must be equal to oneanother. All other things being equal, the first condition would entaila larger value of the capacitance values for the inner layer circuitsthan for the outer layer circuits. In order to obtain the requiredequality of capacitances, it is thus necessary that the dielectricconstant e, of the insulation of the pairs or quads in the inner layersbe smaller than the corresponding dielectric constant e, for the pairsor quads in the outer layers. In other words, it is necessary, if thespac ing of the wires is assumed to be specified, that it be possible togive to the dielectric constants well defined values. The objects of thepresent invention are various arrangements making it possible, duringthe manufactur ing of cables, to adjust the various dielectric constantsintervening in the capacitances of its circuits, while preserving agiven spacing between the wires of the said circuits and the applicationof these arrangements to a balanced circuit such as a carrier currentcable including, for instance, star quads, multiple twin quads or pairs.

The main object of the present invention is a new meth- 0d ofconstruction for a cable allowing the above-explained requirements to befulfilled in a simple way.

According to the present invention, there is provided a telephone cableincluding multiple circuits such as balanced pairs or quads, moreparticularly adapted to high carrier current operation and comprisingarrangements for equalizing the high frequency transmissioncharacteristics of the circuits in the outer layers located near thecable sheath with those of the inner layers located near the center ofthe cable structure, the equalization being effected by decreasing theeffective dielectric constant of the insulation of the circuits in theinner layers with respect to that of the insulation of circuits in theouter layers, characterized in that the individual insulation ofconductors in said outer layers includes an insulating yarn with a givendiameter wound helically about each of said conductors while theindividual insulation of at least part of the conductors in said innerlayers includes two insulating yarns twisted together and then helicallywound around each of the latter said conductors, the diameter of each ofsaid two twisted yarns being substantially equal to half said givendiameter.

Various embodiments of the invention are hereinafter described by way ofnon-limitative examples, and with references to the appended drawings,wherein:

Fig. 1 shows a conductor in a pair or quad of a telephone cable,insulated according to the known art.

Fig. 2 shows, similarly, the cabling of a quad according to knownmethods.

Fig. 3 shows a transversal cross-section of a multiple quad telephonecable.

Fig. 4 shows a conductor in a pair or quad of a telephone cable,insulated according to the invention.

Fig. 5 shows a transversal cross-section of a quad, the conductors ofwhich are insulated according to the invention.

In the insulated conductor represented on Fig. 1, 1 is the copperconductor, 2 is an insulating wire with a diameter a, laid over 1 with awinding pitch p, and 3 is an insulated layer made of an insulating tapewith a width L, a thickness e and laid with a pitch P.

Figure 2 shows a star quad formed by means of four insulated conductors4, 5, 6, 7 as just described. These four conductors are twisted about aninsulating yarn core 8 having a diameter D. When it is desired, for agiven conductor spacing in such a quad, to decrease the dielectricconstant e of the insulation, the various following methods may be used:

(1) Replacing the insulating yarn 2, with a diameter d, by two yarnswith a diameter [1/2 twisted together, which gives an insulatedconductor with the same dimensions but with a more aerated dielectric.

(2) Decreasing the thickness e of the insulating tape and increasingcorrespondingly the diameter of the insulating yarn.

(3) Decreasing the width D of the tape and consequently decreasing theoverlapping of the turns of the said tape.

(4) While preserving the width D of the tape, increasing the pitch P ofthe tape, which consequently decreases the overlapping of the turns ofthe tape.

(5) Replacing the core 8 with a diameter D by a core formed of twotwines with a diameter D/2 twisted together.

The adjustments 2, 3 and 4 should be reversed if, instead of a decreasein e an increase of this constant is desired. In such a case, anincrease in the tape thickness may be obtained by providing two tapesinstead of one only.

The above arrangements apply just as well to paper insulated cables astocables insulated with special in- There is thus found an importantimprovement both in between circuit pertaining to difierent layers andin that of the corresponding difference in propagation time whensulating materials such as rubber, polyethylene, poly- 5 passing fromcables of the former type to cables of the styrene, etc. and may beapplied separately or in comnew type. bination. In another type of cableaccording to the invention The described arrangements may be appliedprimarily, and hereinafter described, all quads were twisted with forequalizing the constants of circuits placed in difditferent pitches, thethree quads in the inner layer ferent layers, but they can also beapplied for equalizing 0 being twisted with shorter pitches than thenine ones the, constants of the individual circuits placed in one in theouter layer. Because of the short pitches used in Sam layer, f theirCharacteristics are in uifi ienfly the inner quads, the mutualequalization of the latter equalized, for instance because of differenttwist pitches would have been imperfect in the absence of other or anyother reason. arrangements. To obviate this drawback the inner layer Byway of example, there are described hereinafter quad with the shortestpitch was built around acore embodiments of a 12 star-quad cable towhich the deconsisting, instead of a yarn of a given diameter D, scribedarrangements are applied. There is represented of two yarns of diameterD/Z twisted together. in cross-section, on Figure 3, a 12 star-quadcable comin a general way, a 12 star-quad cable according to prising alayer of three quads designated by 9 and a the invention may bespecified as follows: layer of nine quads designated by 10, the twolayers being separated by an insulating taping 11. In order Insulatingyam: that the inductances of the pairs in the inner and outer (inner)Zyarns, diameter 0.511 layers beequalized at high frequencies, theconductors flgtfgg lyamdameterkd in the inner quads are closer togetherthan those in the (inner) 1 tape, thickness a outer quads 0011815553ssnasrnaamn nnam"" thcmss In order to make more obvious thepractical advan- 0 1 tages derived from the proposed constructionmethod, fififiigi some results will now be given, certain of whichrelate 1?- to a former cable type and the others to a cable of the(outenm new type according to the invention. The specification for thetwo types of cables and the results obtained are This specificationcorresponds to values comprised, given hereinafter in table form. Asalready mentioned for the coeflicients k, k1, k2, between 1 and 1.5, andfor the parameters which it is desirable to equalize are the k3 between1 and 2. characteristic impedance and the time of propagation r Inpractice, when using a twisted double yarn or twine at high frequencies.In the table the data referring to for the insulation of a conductor,the said double twine the various layers of quads are respectivelydesignated, undergoes a slight crushing under the action of the Windforshort, by inner or outer, according to whether ing of the insulatingtape and other elements placed around they relate to a circuit in aninner or outer layer, the said conductor, and this crushing is slightlymore pro two considered cable types being, generally speaking, A nouncedthan would be the case if, instead of a twisted of similar construction,except for the particular insuladouble twine, a single twine were usedwith twice as great tion processes which are the object of theinvention. a diameter. T o preserve the desired dimensional ratios Inthe table, the average characteristic impedance of between the elementsof the outer layer and those of the the circuits in the difierent layersis given at a frequency inner layer in the cable, it may be necessary touse for of kc./s.; the difference between the average times the former asingle twine with a diameter slightly less than of propagation perkilometer for the two layers expressed twice that of the elements of theabove mentioned double in relative values is given for a frequency of240 kc. s., twine, i. e. to select, for the numerical coefiicient k, avalue as the said frequencies are those generally selected for between0.9 and 1, for instance. It should be understood checking the two saidcharacteristics. The actual values that the above mentioned crushingprevails only in a small of the propagation times are not given, since,for an proportion and does not cause any appreciable increase increasedaccuracy in the measurements, their difierences in the dielectricconstant of the insulation of conductors alone were measured in the testthe results of which using a twisted double twine. are summarized in thefollowing table. This table also The examples described are relative toa 12 star-quad gives, for reference purposes the inductance values perWith two layers. Within the scope of the present invenkilometer measuredat low frequencies (at 0.8 kc./s). tion, similar arrangements may beapplied for equalizing Former cable New cable 1 yarn, diameter, dr 2yarns diameter 0.5d1. 1 yarn, diameter l.1di 1 yarn diameter da.

( 1 tape, thickness e1 1 tape, thickenss or.

1 tape, thickness 1.1a. Do. Quad core (Inner) 1 wire; diameter D1- 1wire; diameter D.

. do Do.

(Outer) Inducltlgililce at 0.8 ke./s. H/km:

25.63. (Outer) 25.76. Impedance Z at 120 k (Inner 173.14. (Outer) 174.8174.35. The relative ditierences in the time of propagation perkilometer t and in the impedance Z are, between pairs in differentlayers:

At/t in thousandths at 240 kc.ls -30 A Z/Z in thousandths at 120 ke./ 8

the constants of the circuits of cables with multiple layers consistingof star quads, multiple twin quads or pairs.

What I claim is:

In a telephone cable; the combination of multiple circuits of thebalanced star-quad type adapted for high frequency current transmissionand arranged in inner and outer layers, a sheath around said outerlayers, the quads in said inner layers having shorter twist pitches thanthe quads in said outer layers, each of said quads including aninsulating core, said insulating cores of the quads in said outer layersbeing formed of an insulating yarn of a predetermined diameter, saidinsulating cores of at least part of the quads in said inner layersbeing formed of an assembly of two insulating yarns twisted together andeach having a diameter substantially equal to one-half saidpredetermined diameter, individual insulation for the conductors of thequads in said outer layers including an insulating yarn with a givendiameter wound helically about each of said conductors in the outerlayers, and individual insulation for at least part of the conductors ofthe quads in said inner layers including two insulating yarns twistedReferences Cited in the file of this patent UNITED STATES PATENTS475,648 Wesslav May 24, 1892 1,987,442 Harris Jan. 8, 1935 2,116,267Klimmer May 3, 1938 2,116,268 Klimmer May 3, 1938 FOREIGN PATENTS788,220 France Oct. 7, 1935 524,674 Great Britain -2 Aug. 12, 1940

